1. Field of the Invention
This disclosure relates generally to image sensors, and in particular but not exclusively, relates to backside illumination CMOS image sensors.
2. Background Art
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The demands of higher resolution and lower power consumption have encouraged further miniaturization and integration of these image sensors. As a result, technology used to manufacture image sensors, for example, CMOS image sensors (“CIS”), has continued to advance at a great pace.
FIG. 1 is a circuit diagram illustrating circuitry of four-transistor (“4T”) pixel cell 100 of a conventional image sensor device. Pixel cell 100 includes photodiode PD 110, transfer transistor TX 115, reset transistor RST 125, source follower (“SF”) transistor SF 130, and row select transistor SEL 135. During operation, transfer transistor TX 115 receives a transfer signal provided via TX line 140, which transfers charge accumulated in PD 110 to floating diffusion node FD 120. RST 125 is coupled between RST power line 150 and FD 120 to reset the pixel (e.g., to discharge or charge FD 120 and/or PD 110 to a preset voltage) under control of a reset signal provided via RST line 160. FD 120 is also coupled to control the gate of SF 130. SF 130 is coupled between SF power line 180 and SEL 135. SF 130 operates as a source-follower providing a high impedance connection to FD 120. Under control of a select signal which is provided via row select line 170, SEL 135 selectively provides an output of pixel 100 to column readout line or bit line 190.
PD 110 and FD 120 are reset by temporarily asserting the reset signal of RST line 160 and the transfer signal of TX line 140. An image accumulation window (exposure period) is commenced by de-asserting the transfer signal of TX line 140 and permitting incident light to charge PD 110. As photo-generated electrons accumulate on PD 110, its voltage decreases. The voltage or charge on PD 110 is indicative of the intensity of the light incident on PD 110 during the exposure period. At the end of the exposure period, the transfer signal of TX line 140 is asserted to allow an exchange of charge between PD 110 and FD 120, and hence the gate of SF 130. The charge transfer causes the voltage of FD 120 to drop by an amount which is proportional to photogenerated electrons accumulated on PD 110 during the exposure period. This second voltage biases SF 130 which, in combination with the select signal being asserted on row select line 170, drives a signal from SEL 135 to bit line 190. Data is then readout from pixel cell 100 onto bit line 190 as an analog signal.
Many current semiconductor image sensors today are frontside illumination (FSI) devices. That is, they include imaging arrays that are fabricated on the frontside of a semiconductor wafer, where light may be received at the imaging array from the same frontside. A FSI image sensor has disposed on or over the frontside of a silicon wafer a metal stack, where the metal stack includes metal layers (e.g. separated by intermetal dielectric layers) to variously provide signal lines to respective pixel elements of a pixel array.
In existing FSI technology, the routing and utilization of traces in a metal stack is constrained by the fact that light received by a given pixel cell first has to pass through the metal stack. For example, in a pixel array having pixel cells such as pixel cell 100, a given row (not shown) of pixel cells share TX line 140 and RST line 160. However, use of any additional row-wise lines by pixel cell 100 is constrained due to a need to accommodate the receipt of light by PD 110. Accordingly, rather than also being shared across such a row, SF power line 180 is coupled to one pixel cell of the row, but not to the other pixel cells of that row. Rather, SF power line 180 only provides current to source follower transistors of pixel cells (not shown) which, as compared to such a row of pixel cells, form an orthogonal column of pixel cells. Such a column of pixel cells share bit line 190, and so cannot be read out concurrently on a column-wise basis.
Generally speaking, miniaturization in image sensors results in smaller photodiodes which generate smaller amounts of charge for smaller amounts of incident light, where signals of smaller voltage and/or current levels are in turn generated for representation of the captured image. The effective generating and processing of such signals poses one challenge for next-generation image sensors.